A prior art apparatus is shown in FIG. 14 which comprises an upright shell 1 filled with a pulverizing medium b such as steel balls, and a vertically extending screw shaft 2 mounted in the shell 1. The material m to be pulverized is fed into the shell 1 with the screw shaft 2 in rotation to circulate the material m in the shell 1. When the material m is pulverized into a particulate product c having a desired particle size by the friction with the pulverizing medium b and between the particles of the material, it is entrained in the flow of air or water through the shell 1 to leave the shell. Other parts shown are a circulating fan 3, a product collector 4 such as a bag filter and a cyclone separator, and a rotary valve 5 for feeding the material m to be pulverized into the shell 1.
With this type of pulverizing apparatus, the screw blade tends to wear remarkably at its bottom portion 6a because the screw cuts into the material m and the pulverizing medium b at this portion. To protect that portion from wear, it has been a common practice to fuse a ceramic material or a wear-resistant alloy to the portion or form the portion of molded portions made of such materials.
But, it is troublesome and time-consuming to provide such a wear protective means. Its maintenance and replacement is also troublesome.
The screw shaft of Japanese Unexamined Utility Model Publication 59-131241 see (FIG. 15) or Japanese Examined Patent Publication 39-12187 see (FIG. 16) is a screw shaft 2 having a disk 7 coaxially secured to its bottom end. Thus the screw shaft is well protected against wear. The disk 7 itself is less liable to wear because it is not adapted to cut into the material m and the pulverizing medium b but come into contact with them on a flat surface.
But if the screw shaft 2 should run out of true, thus inclining the disk 7, the frictional resistance acting on the periphery of the disk 7 will be out of balance. This will increase the degree of runout of the screw shaft 2, thus imparing the pulverizing efficiency. If the screw shaft runs too much out of true, the operation of the machine might be impossible.
In the latter of the above-described publications, the disk 7 is provided on its bottom surface with radial ribs 8 having a triangular shape. The screw shaft 2 might run out of true violently if there is a substantial difference among the frictional resistances acting on these ribs.
Further, with such prior art pulverizers, part of the material m fed into the shell through its inlet 9a tends to be discharged directly to its outlet 9b without being pulverized. Thus it is necessary to provide a collector 9 such as a cyclone separator between the outlet 9b and a product collector as shown in the drawing to collect the product c flowing out of the outlet 9b and feed it back into the shell 1. If the fluid in the shell 1 is liquid, a collector 9 such as a settling classifier has to be provided as shown in FIG. 17 to collect only the coarser product c and feed it back into the shell 1 by means of a pump.
The provision of the collector 9 will not only make the machine bulky but also complicate the fluid control. Moreover, it is necessary to increase the driving force to drive the collector 9 which does not serve to pulverize the material. Thus the provision of the collector will increase the running cost and impair the pulverizing efficiency in comparison with the case in which no shortcircuiting of material takes place.
Another problem with a prior art pulverizer is that the pulverizing medium b tends to flow in a rather simple, concerted manner because only one screw shaft 2 is provided in the shell 1. The material m is thus liable to be discharged unpulverized from the shell, resulting in an increase in the content of coarser particles in the product c.